You might be familiar with the sinking feeling that comes when the dreaded engine warning light flashes up. It’s usually an amber light that’s located in or around the instruments behind the steering wheel and appears when the car’s sensors detect a problem with your engine or another part of the running gear.
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In most cars, a dashboard warning light is usually the first indication that something may be wrong in a petrol or diesel car. Hybrid and electric cars also have similar warning lights. The warning is designed to alert you to a potential issue with your car as early as possible to prevent further damage to the engine or other related parts. The amber engine warning light is the most common of these warnings and is sometimes also referred to as the ‘check engine’ light or ‘engine management’ light.
What is an engine management warning light?
The engine management warning light is different from the other lights on your dashboard, as it doesn’t alert you to a specific fault. Unlike the coolant temperature warning or low oil level warning lights, the engine management light (EML) is more of a general signal that something is wrong and can be triggered for a number of reasons.
A flashing or continuous warning light can show if the engine isn't running correctly. An EML – usually presented to depict a stylised engine block, but sometimes taking the form of a 'check engine' message – will illuminate as a prompt to seek further assistance. In the majority of cases, the car will need to be connected to diagnostic equipment to determine exactly where the fault lies. This can be done at any good service centre or at the roadside if your breakdown assistance provider has one.
Some older cars might flash the ‘check engine’ light a specific amount of times, depending on what the issue is. In these cases, counting the amount of times it flashes before a short pause may refer to an error code which can be found in the car’s manual to tell you what’s wrong.
In many cases, the light operates on a colour-coded basis. An amber EML doesn't necessarily point to an unfolding emergency – you should be able to continue on your way and seek advice on reaching your destination. A red light, though, suggests that you should pull over where it's safe to do so and arrange for your car to be recovered to a garage. Driving further with a red light could potentially cause expensive damage. Engine warning light MOT issues can stem from faults in the emissions control system, so it's worth seeking assistance sooner rather than later.
Will my car pass its MOT with an engine warning light?
In 2018, stricter MOT rules were introduced meaning that any car undergoing an MOT test with an illuminated engine management light (whether it’s a red or amber warning) would automatically fail.
Prior to taking your car for an MOT test, it’s worth checking that all warning lights go out when you start the engine; it’s normal for the engine management light to illuminate with the ignition prior to starting, but it should always go out once the car is running.
How to reset the engine management light
While the documentation for some older cars provided instructions on how to check and reset engine warning lights, it's not usually possible now cars have grown more sophisticated; these days, you’ll usually need to take the car to a repair specialist who can diagnose the issue and reset the light. You can purchase your own vehicle diagnostic readers, with prices varying depending on brand and capability. It's well worth paying for an accurate diagnosis, though, as you may be neglecting a vital issue that requires a full and effective repair.
Causes of engine management light
The list of possible triggers for an EML is a long one, but here are five of the most frequent causes:
Emissions system fault
Reducing engine exhaust emissions is one of the most demanding tasks a manufacturer faces when designing a new car. Nobody wants to pollute the environment excessively, but equally, nobody wants a car that’s horribly stifled and can’t be driven in a normal fashion. To ensure cars are still enjoyable to drive, while preventing or reducing the emission of noxious gases, manufacturers employ a variety of control systems to manage what comes out of the exhaust.
One key component is the oxygen sensor, with many cars having more than one. This measures the amount of unburnt oxygen passing through the exhaust of the car, which can indicate whether the engine is burning too much or too little fuel. If too little oxygen leaves the exhaust, the engine will be running ‘rich’ – taking too much fuel and not enough air into its cylinders.
An EML triggered by the oxygen sensor can mean the car is running too rich or too lean, or can be caused by a fault with the sensor itself. To put this fault right may involve the reprogramming or recalibration of the engine control unit (ECU) or, sometimes, the replacement of the oxygen sensor.
It is worth noting that in a few years’ time, cars that do not pass a low-emissions test may instantly fail their MOT. The new Euro 7 emissions standard is due to bring in rules that subject cars to regular emissions screening, deeming those that fail to filter out a sufficient amount of harmful gases as unroadworthy.
Blocked diesel particulate filter or differential pressure sensor fault
Some diesel-powered cars have a specific warning light for their diesel particulate filter (DPF), whereas others don’t and rely on the EML providing a warning instead. The latter is sometimes also illuminated if the car detects a fault with the differential pressure sensor, which determines how effectively the DPF is performing.
In simple terms, this is a device that measures the ability of gas to pass along the exhaust, which includes passing through the diesel particulate filter. This is a feature fitted to diesel cars that removes carcinogenic particles from the exhaust, preventing them from polluting the atmosphere.
These particles take the form of a sooty deposit, and over time the filter becomes so heavily clogged with particles that exhaust gases struggle to pass through it. When the differential pressure sensor determines this point has almost been reached, the engine begins a process called DPF regeneration.
This involves extra fuel being burnt by the car in order to increase the temperature of the DPF. When it gets hot enough, the particulates are burnt, or oxidised, so they can pass harmlessly through the exhaust as fine ash.
The EML (or sometimes a separate DPF light) can illuminate if this regeneration process fails to occur. Diesel cars that are only used for occasional local journeys don’t ever reach ordinary operating temperature, which is one of the conditions required before regeneration can occur. If the DPF can’t regenerate, it becomes blocked, which means the engine can’t breathe properly. If you own a diesel car that you typically only use for short trips, we recommend taking your car out for longer drives as and when possible to avoid this type of issue developing - or swapping to a car that runs on petrol or electricity.
However, a faulty differential pressure sensor can also create the same symptoms. If it’s producing an incorrect reading – or none at all – the ECU doesn’t accurately know how full the car’s DPF is. It can then sometimes trigger a low-power (or limp-mode) mode to avoid the risk of damage until the fault can be investigated.
If the DPF filter has become blocked the workshop should be able to perform a ‘forced regeneration’ using diagnostic equipment to communicate directly with the engine. Its first task, though, is to make sure the differential pressure sensor is providing accurate readings.
Mass airflow sensor
The mass airflow sensor (sometimes referred to as a hot-film mass airflow sensor, air mass sensor, mass flow sensor or MAF) is vital for the efficient running of an engine. In its combustion chambers, an engine burns a mixture of fuel and air. The ECU (Electronic Control Unit) uses data from the mass airflow sensor to calculate how much fuel to add to the combustion chambers to match the amount of air flowing into the engine.
Without this information, all it can do is guess. A modern ECU will always have a ‘safe’ set of parameters it can adopt to keep the car running if a fault arises. This is often referred to as ‘limp-home mode’ and can sometimes be triggered by inaccurate or missing data from the mass airflow sensor. If this is the case, you may notice a loss of power and/or lack of responsiveness when you depress the accelerator.
The mass airflow sensor is mounted towards the top of the engine, where it’s in the path of air that has just passed through the engine’s air filter. An incorrectly installed (or missing) air filter can damage the sensor by allowing harmful particles to make contact with it.
Alternatively, a clogged air filter not letting enough air through will trigger a warning that the engine is being forced to run ‘rich’. Whether the signal given by the mass airflow sensor is missing or outside normal parameters, the effect will be the same – an engine warning light.
Ignition system fault
Although many people refer to the starter button or key as their car’s ignition – it’s short for ‘ignition switch’ – a car’s ignition system is far more than just how it switches on and off. The ignition system is responsible for actually burning or ‘igniting’ the fuel that makes an engine work.
In a petrol car, a mixture of fuel and air, in the form of vapour, are forced into the combustion chambers, where it’s rapidly burnt in a small, controlled explosion, started by a spark. This spark is provided by a spark plug, which is a component of the ignition system.
In a diesel car, the same basic process occurs, but no spark is required for combustion to take place. Instead, the mixture of diesel fuel and air is ignited by the compression of the piston acting on it. However, when you start a diesel engine on a cold day, a component called a glow plug heats the combustion chamber before the fuel/air mixture is added, making combustion easier.
Power for the glow plugs in a diesel engine and the spark plugs in a petrol is provided by the car’s ignition system. In petrol cars, a very high voltage is needed to create the spark that ignites the fuel. This is provided by the engine’s coils. Many modern cars have a coil for each cylinder, and if this coil develops a fault, the fuel in that cylinder won’t burn effectively, which not only causes the engine to run roughly or down on power, but can also cause damage to your exhaust system’s catalytic converter.
While glow-plug failure is usually indicated by a separate warning light in a diesel car (usually a swirling line, resembling an insect with antennae), problems with the spark plugs and coils of a petrol car and the wiring between them are usually covered by the EML. Faults like this are commonly referred to as a ‘misfire’ and repair often involves the replacement of one or more spark plugs, spark plug leads or coils.
Loose petrol or diesel filler cap
The last entry in our list can be one of the most frustrating, although it usually comes as something of a relief when it’s found to be the reason for your EML illuminating.
Next time you open your fuel filler cap, listen out for a rush of air. This is caused by a build-up of air pressure or vacuum in the fuel tank. Far from being a simple container with a supply pipe to the engine, a fuel tank is a complicated, pressurised system in itself. Fuel is pumped out of the tank towards the engine, and as the fuel is pumped out, unless air takes its place, the tank would simply crumple up like an empty drinks can.
The fuel tank has a vent that allows metered amounts of air from outside to replace the fuel as it’s pumped out of the tank. Air is allowed into the tank based on readings from a fuel tank air-pressure sensor, which is able to account for things like temperature that can also have an effect on the air pressure in the tank. If the fuel tank pressure sensor is unable to provide an accurate reading, it’ll usually trigger an engine warning light.
However, the readings from the sensor can only be reliable if the fuel tank system is 100% pressure tight, with air only permitted to enter and leave the tank through the controlled vents. One common cause of an air leak into – or out of – the petrol tank, is through a fuel filler cap that doesn’t properly seal.
This is one of the cheapest-to-fix faults that can possibly trigger an EML, with a replacement fuel filler cap often typically costing around £10.
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